ATM communications system and method

ABSTRACT

There is disclosed a method and a system for multiplexing data from a plurality of user data sources across an ATM adaption layer type-2 connection, in which a multiplexed trunk group extends across a plurality of common part sub-layer protocol data unit (CSU-PDU) mini-cells, and across a plurality of ATM cells. Large trunk groups are assembled by use of a single bit continuation indicator in the service specific convergence sub-layer header (SSCS) of successive CPS-PDU mini-cells. A packet payload type field (PPT) of the common part sub-layer (CPS)/service specific convergence sub-layer (SSCS) is used to indicate timing of changes in number of user data sources in a trunk group and provides for robust error recovery on loss of a single CPS-PDU mini-cell.

FIELD OF THE INVENTION

The invention relates to digital communications networks, andparticularly, although not exclusively to an arrangement and method fortransmitting multiplexed multi-user asynchronous transfer mode (ATM)traffic across such communications networks.

BACKGROUND TO THE INVENTION

The known asynchronous transfer mode (ATM) transmission technique is amodem telecommunications switching technique which is able to switchconnections for a wide range of different data types at a wide range ofdifferent bit rates. ATM technology provides a flexible form oftransmission which allows various types of service traffic data, e.g.voice data, video data, or computer generated data to be multiplexedtogether onto a common physical means of transmission. Currently,several trends are encouraging the widespread introduction of ATM; forexample the availability of high speed, low error rate communicationlinks between switching centers, an availability of technology todigitize video and speech, and pressure to reduce operating costs byintegrating previously separate telephony and data networks. ATMtechnology allows speech data, video data and inter-computer data to becarried across a single communications network. The information carriedin each of these services is reduced to digitized strings of numberswhich are transmitted across such a communications network from point topoint.

A method of switching synchronous transfer mode cells in a circuitemulated ATM switch using a layered protocol model is described inspecification No. WO-95-34977. A method of transferring ATM microcellsin a telecommunications system is described in specification No.WO-96-34478.

Referring to FIG. 1 herein, there is shown schematically a portion of acommunications network comprising first and second node devices 100, 101respectively linked by a communications link 100. Transport of ATM datacommunications traffic is made across the communications link 102between the first and second node devices, which may be for exampleswitches 101, 102. Digitized data is received from customer equipmentsuch as telephones, computers, faxes, modems and video broadcastapparatus in the form of frames of digitized signals at transmittingnode, e.g. switch 100. The frames can either be of variable length orfixed length, and may arrive at the switch at a variable rate; or at afixed rate. The frames of data arriving at the switch are packaged intoATM data cells 103, which have a fixed number of bytes. Transport of ATMcells between node devices is handled by the node devices operating inaccordance with the ATM protocol corresponding to the InternationalStandards Organization (ISO) Open Systems Inter-connexion (OSI)architecture, layers two and three⁽¹⁾. Packaging of the incoming dataframes received asynchronously from the customer equipment is handled bythe switches operating in accordance with ATM adaptation layer (AAL)protocols which segment the arriving frames of data into payload data ofthe ATM cells at the transmission node, and reassemble the payload datainto frames at the destination node 102. The ATM adaptation layercorresponds to layer four of the OSI model. Equipment operating inaccordance with the ATM adaptation layer protocols are capable ofstructuring incoming data in different ways, to suit different servicetypes, e.g. video data, computer generated data, voice data. Manydifferent service types can be implemented by the ATM adaptation layersimultaneously.

The basic reason for having ATM cells is that they have a fixed length.Fixed length cells are easier for hardware to handle than variablelength frames. The ATM adaptation layer packages various types of dataof variable length or fixed length frame type into the fixed length ATMcells for transport between physical devices. Because the ATM celllength was historically selected to accommodate various types oftraffic, fixing the length of the ATM cell involved difficult decisions,and the final length of ATM cell selected is not perfect for each typeof data carried. The ATM cell comprises a header portion which carriesrouting information and other housekeeping information necessary for theoperation of the ATM network, and a payload portion which carries theactual data traffic. To transfer delay sensitive services such asspeech, it is important that the ATM cell be reasonably short in orderto avoid unacceptably long delays in filling the cell payload portionbefore transmitting the cell across the network. On the other hand, forother types of traffic such as computer to computer file transferslonger cells are more efficient, since the proportion of availabletransmission bandwidth taken up by the ATM cell header compared to thedata payload of the cell is reduced. For delay insensitive traffic, theoverhead of the housekeeping information sent in the header of each ATMcell would be relatively large if short cells were to be used. Thus, thechoice of ATM cell size is a compromise and is settled at a length of 53octets, comprising 48 octets of data payload (the ATM Service Data Unit,ATM-SDU) and a 5 octet header for transmission of housekeeping protocolinformation, as shown schematically in FIG. 2 herein. The protocolheader in the ATM cell constitutes approximately 10% of the whole cell.This size of ATM cell introduces a delay in transmission of data whichis significant for types of data having a low data rate, for examplespeech data. For example for a conventional 64 kilobits per second(kbit/s/s) voice data traffic, normal speech data samples are convertedinto one octet of digital data every 125 microseconds (μs). Thus, 48×125μs=6000 μs are required to fill the 48 data octets of an ATM cellpayload. This introduces a 6 millisecond (ms) delay to each celltransmitted, in addition to two network switching delays one from eachswitch, and transmission delays across the network. For speech services,it is important to have an effectively constant delay between source anddestination of a call, and the delay must be reasonably short. Largevariations in delay produce broken sound effects, and make voice signalsunintelligible to a service user. Long delays, for example thosesometimes encountered on transatlantic satellite links, make two-wayconversation awkward. In general, a conventionally accepted maximumone-way delay for speech data is 25 ms. Delays longer than this, as wellas making the speech service unacceptable to users, also requirecomplicated and expensive echo suppression equipment, which has theadditional disadvantage of introducing noise. Thus, the conventional 53octet ATM cell is not ideal for 64 kbit/s voice data traffic. However,with the advent of mobile telecommunications systems, normal 64 kbit/ssampled voice signals are compressed using code compression algorithms,resulting in transmission data rates as low as 4 kbit/s. Under thesecircumstances, the delay introduced in filling a full ATM cell may be ashigh as 96 ms, an unacceptably high delay.

In view of the above delays and to accommodate different data traffictypes, the ATM adaptation layer (AAL) is split into a number ofsub-layers. A first sub-layer, the known AAL-type 1 layer is aimed atconstant bit rate services. The currently developing, and not yetfinalized AAL-type 2 layer (formerly known in Europe as AAL-type 6, andelsewhere as AAL-CU) allows multiple variable length sub-cells, calledmini-cells to be carried within one ATM cell. An object of AAL-type 2 isto support all services which require the multiplexing of informationfrom multiple user data sources into a single ATM connection. TheAAL-type 2 protocol, which breaks the basic rule of ATM that all cellsbe of fixed length, is aimed at being expedient for carrying low speeddata where the delay caused by waiting for a full ATM cell to fill istoo long, and the overhead of carrying an incomplete ATM cell is toogreat. However, the implementation of this layer is incomplete and therestill remains a requirement for a method of transmitting data from amultiplicity of sources, including low bit rate data, over ATM networksin an efficient manner.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an improved method fortransmitting delay sensitive data from a plurality of user data sourcesover a communications network.

Another object of the present invention is to provide a method oftransmitting multi-service data from a range of different data sourcesover an ATM network in an efficient manner.

Another object of the present invention is to provide a robust method oftransmitting large trunk groups of data of a plurality of data usersources, over a communications network.

According to one aspect of the present invention there is provided amethod of transmitting data in a communications network, said methodcomprising the steps of:

receiving a plurality of data samples comprising at least one said datasample received from each of a plurality of user data sources;

multiplexing said plurality of data samples into a data payload of atleast one asynchronous transfer mode mini-cell;

packetizing said mini-cell(s) into at least one asynchronous transfermode cell; and

transmitting said asynchronous transfer mode cell(s).

Preferably each said mini-cell carries data from a plurality of userdata sources.

According to a second aspect of the present invention there is provideda method of communicating user data of a plurality of user data sourcesover a communications network, said method comprising the steps of:

multiplexing a respective data sample from each of a plurality of userdata sources to produce a frame of user data, said frame containing dataof each of said plurality of user data sources; and

assembling said frame into a plurality of data payloads of acorresponding plurality of asynchronous transfer mode mini-cells.

Said frame may be of a length greater than a payload of a single saidmini-cell.

A said frame may be partitioned to run consecutively across a pluralityof said data packet payloads.

Preferably said method comprises the step of:

assembling a respective protocol header to each of said mini-cells, saidprotocol header comprising a continuation indicator signal indicatingwhether or not said frame continues beyond a length of said mini-cell.

Preferably said continuation indicator signal comprises a single bitfield.

Preferably a said mini-cell comprises an asynchronous transfer modeadaptation layer-type 2-common part sub-layer packet.

Preferably said continuation indicator signal comprises an asynchronoustransfer mode adaptation layer-type 2 header.

Preferably said continuation indicator signal comprises an asynchronoustransfer mode adaptation layer-type 2 service specific convergencesub-layer field.

According to a third aspect of the present invention there is provided amethod of communicating user data of a plurality of user data sourcesover a communications network, said method comprising the steps of:

signaling an asynchronous transfer mode adaptation layer type-2connection;

multiplexing user data of said plurality of user data sources into atrunk group frame;

assembling said trunk group frame into a payload of at least oneasynchronous transfer mode variable length cell;

transmitting said asynchronous transfer mode cell(s) across saidnetwork.

According to a fourth aspect of the present invention there is provideda method of communicating user data of a plurality of user data sourcesover a communications network, said method comprising the steps of:

multiplexing a data sample from each of a plurality of user data sourcesto produce a frame of user data, said frame containing data of each ofsaid plurality of user data sources; and

assembling said frame into a data payload of at least one asynchronoustransfer mode mini-cell.

Preferably said method comprises the step of:

including a protocol header signal in a said mini-cell, to indicate achange in number of user data sources who's data is assembled into asaid at least one mini-cell.

Preferably said method comprises the step of using a packet payload typefield of a service specific convergence sub-layer header of anasynchronous transfer mode mini-cell to indicate change of number ofsaid user data sources who's data is carried in a series of saidmini-cells.

Preferably said method comprises the step of using a packet payload typefield of an asynchronous transfer mode mini-cell header to signal timingof a change of a number of said multiplexed users.

Preferably said mini-cell comprises an ATM adaptation layer type-2mini-cell.

According to a sixth aspect of the present invention there is provided amethod of communicating user data to a plurality of user data sources ofa communications network, said method comprising the steps of;

multiplexing data of a first plurality of user data sources into a firstdata group having a first group size;

assembling said first data group into a data payload of a first set ofat least one mini-cell;

multiplexing data of a second plurality of user data sources into asecond data group having a second group size;

assembling said second data group into a data payload of a second set ofat least one mini-cells,

wherein each said mini-cell comprises a respective protocol header, saidprotocol header arranged to indicate a change in group size betweensuccessive mini-cells.

According to a seventh aspect of the present invention there is provideda method of transmitting data in a communications network, comprisingthe steps of:

receiving a plurality of data samples comprising at least one said datasample received from each of a plurality of user data sources;

multiplexing said plurality of data sources into a trunk group;

establishing a trunk group connection using an asynchronous transfermode adaptation layer-type 2 negotiation procedure;

signaling additions or subtractions of users in the trunk group byincorporation of signals contained within an asynchronous transfer modeadaptation layer-type 2 protocol header, whilst leaving saidasynchronous transfer mode adaptation layer-type 2 trunk groupconnection intact.

According to an eighth aspect of the present invention there is provideda method of communicating user data of a plurality of user data sourcesacross a communications network comprising a plurality of transmittingentities and receiving entities, said method comprising the steps of:

signaling to a said receiving entity to create a trunk group connection;

multiplexing data of said plurality of user data sources into a trunkgroup data frame;

signaling to said receiving entity a length of said trunk group dataframe;

assembling said trunk group data frame into a plurality of asynchronoustransfer mode adaptation layer type-2 mini-cells; and

signaling a change in length of said trunk group data frame in anasynchronous transfer mode adaptation layer type-2 mini-cell header.

Preferably said step of signaling a length of said trunk group dataframe comprises signaling by an asynchronous transfer mode negotiationprocedure protocol.

Preferably said step of signaling a change in length of said trunk groupdata frame comprises signaling within an asynchronous transfer modeadaptation layer type-2 payload packet type field.

According to a ninth aspect of the present invention there is provided amethod of implementing changes in data payload of an asynchronoustransfer mode adaptation layer type-2 mini-cell by decoding a payloadpacket type field of said mini-cell in accordance with the followingsteps:

decoding a received packet payload type signal 10 as indicating amini-cell contains a new data payload containing data from a differentnumber of user data sources compared with a previously receivedmini-cell.

Preferably said method comprises the step of decoding a received packetpayload type field signals as follows:

decoding a first mini-cell having packet payload type field 00 followedby an immediately succeeding second mini-cell including packet payloadtype signal 00 as indicating no change in a number of user data sourcesin a data payload of said mini-cells.

Preferably said method comprises the step of decoding a received packetpayload type field signals as follows:

decoding a first mini-cell having packet payload type field 01 followedby an immediately succeeding second mini-cell including packet payloadtype signal 01 as indicating no change in a number of user data sourcesin a data pay load of said mini-cells.

Preferably said method comprises the steps of:

in an asynchronous transfer mode adaptation layer-type 2 connection,decoding a received packet payload type signal 00 of a first mini-celland a received packet payload type signal 01 of a next receivedmini-cell as indicating a loss of data.

Preferably said method comprises the steps of:

in an asynchronous transfer mode adaptation layer-type 2 connection,decoding a received packet payload type signal 01 of a first mini-celland a packet payload type signal 00 of next received mini-cell asindicating a loss of data.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect, there will now be described by way of exampleonly, specific embodiments, methods and processes according to thepresent invention with reference to the accompanying drawings in which:

FIG. 1 shows schematically a portion of a communications networkcomprising first and second node devices;

FIG. 2 shows schematically the ATM cell size comprising a length of 53octets;

FIG. 3 illustrates conceptually a communications network hardwarecomprising first and second node devices connected by a communicationslink device, the node devices acting as receiving and transmittingentities for transmission of communications data;

FIG. 4 illustrates a transmission series of asynchronous transfer modecells, containing a plurality of asynchronous transfer mode adaptationlayer type-2 mini-cell (common part sub-layer packets);

FIG. 5. illustrates schematically a layout of an asynchronous transfermode common part sub-layer packet mini-cell;

FIG. 6 illustrates schematically an asynchronous transfer mode mini-cellcommon part sub-layer protocol header, and a mini-cell data payload;

FIG. 7 illustrates schematically a start field header in an asynchronoustransfer mode mini-cell data payload;

FIG. 8 illustrates schematically assembly of data of a plurality of userdata sources into a plurality of CPS packet mini-cells in accordancewith a single channel adaptation (SCA) method;

FIG. 9 illustrates schematically assembly of data from a plurality ofuser data sources into a plurality of CPS packet mini-cells inaccordance with a multiple channel adaptation method (MCA) according toa specific method of the present invention;

FIG. 10 illustrates schematically a method of assembling a CPS packetmini-cell including a data payload comprising a multiplex of data of aplurality of user data sources, and assembly of a common partsub-layer/service specific convergence sub-layer (CPS/SSCS) header inaccordance with a multiple channel adaptation method;

FIG. 11 illustrates schematically a plurality of CPS packet mini-cellscontaining a data payload of a single trunk group data frame comprisingmultiplexed data from a plurality of user data sources;

FIG. 12 illustrates schematically a series of CPS packet mini-cellstransmitted across a communications network in accordance with specificmethods of the present invention, and a result of decoding signalinginformation comprising the transmission, in accordance with furtherspecific methods according to the present invention;

FIG. 13 illustrates schematically in overview, a signaling processaccording to a specific method of the present invention;

FIGS. 14 to 16 illustrate schematically in overview, a signal decodingprocess comprising specific methods of the present invention;

FIG. 17 illustrates schematically a decoding sub-process according to aspecific method of the present invention;

FIG. 18 illustrates schematically another decoding sub-process accordingto a specific method of the present invention;

FIG. 19 illustrates schematically another decoding sub-process accordingto a specific method of the present invention;

FIG. 20 illustrates schematically a method of multiplexing long datastructures into a plurality of mini-cells; and

FIG. 21 illustrates a method of multiplexing relatively short datastructures into a plurality of mini-cells.

DETAILED DESCRIPTION OF THE BEST MODE FOR CARRYING OUT THE INVENTION

There will now be described by way of example the best mode contemplatedby the inventors for carrying out the invention. In the followingdescription numerous specific details are set forth in order to providea thorough understanding of the present invention. It will be apparenthowever, to one skilled in the art, that the present invention may bepracticed without using these specific details. In other instances, wellknown methods and structures have not been described in detail so as notto unnecessarily obscure the present invention.

Referring to FIG. 3 herein, there are shown first and second ATM nodedevice, 300, 301 respectively between which are transmitted data signalsin the form of ATM cells 302 which are transmitted across a physicaltransmission medium 303 between the node devices single user ormulti-service data from a plurality of single service or multi-servicedata sources, for example speech data 304, video data 305, and computergenerated data 306 are input into the node devices 300, 301 andincorporated into the plurality of ATM cells 302. FIG. 3 illustrates asimple form of physical network comprising two node devices and one linkdevice, in order to illustrate principles of the best mode for carryingout the invention without unnecessarily obscuring the invention. It willbe appreciated by the skilled reader that methods and processesaccording to the invention will be applicable to highly complexcommunications networks.

Referring to FIG. 4 herein, there is shown schematically a series of twoconventional ATM cells, each containing a plurality of conventional ATM,CPS Protocol Data Unit (CPS-PDU) mini-cells MC1-MC6. A single mini-cell400 comprises a 3 byte mini-cell header 401, and a variable lengthmini-cell payload 402 for carrying data traffic. In AAL-2 one or moremini-cells may comprise the payload of an ATM cell, occupying a maximumof 48 octets of the ATM cell, the remaining 5 octets of the ATM cellbeing reserved for the ATM cell header. An ATM mini-cell may cross anATM cell boundary. For example in FIG. 4 first ATM cell, cell 1 containsmini-cells MC1, MC2 and a portion of a further mini cell MC3. Theremaining portion of the mini-cell MC3 occupies a payload of second ATMcell, cell 2.

In AAL-type 2 the mini-cell header 401 is split into two parts; firstlya common part sub-layer (CPS) packet header and secondly a servicespecific convergence sub-layer (SSCS) packet header. The common partsub-layer includes basic control information which is common to allmini-cells transmitted, such as a user identification (UID) indicatingthe number of users of a virtual channel, (an ATM virtual channel canhave up to 256 individual users); a length indicator (LI) fieldindicating the length of the mini-cell payload, the mini-cells beingcapable of having variable length payload; and a cyclic redundancy data(CRC) field to protect the content of the mini-cell header againsterrors. The mini-cell header is of length 3 bytes. Within the mini-cellheader there are left five free bits allocated for the service specificconvergence sub-layer (SSCS) packet header information. The servicespecific convergence sub-layer packet header constitutes a userdefinable field, such that users can adapt the service specificconvergence sub-layer header to tailor the ATM mini-cell to their ownrequirements. Users have an option to place service specific convergencesub-layer information as part of the mini-cell payload 400, or can placethe SSCS information in the vacant 5 bits in the mini-cell header 401.Thus, for example if a user wanted to include digital error protectioninformation for a service, this could be included in the payload of themini-cell. In FIGS. 5, 6 and 7 herein, there is shown schematically the3 byte mini-cell header, comprising the common part sub-layer (CPS), andthe five free bits reserved for the service specific convergencesub-layer (SSCS), and also positioning of elements of the servicespecific convergence sub-layer at a location 501 in the mini-cellpayload 400.

In the AAL-type 2, ATM cells are filled with CPS-PDU mini-cells, suchthat data from different users fill respective different mini-cells oneuser per mini-cell. Thus, using a single channel adaptation (SCA) methodfor example the from three different users can be transmitted in asingle ATM cell, by incorporating three separate mini-cells as thepayload of the ATM cell, each mini-cell containing data from arespective user data source. Thus, the ATM mini-cells can be transmittedasynchronously within the ATM cell structure, and a number of users maybe multiplexed into the payload of an ATM cell, using a plurality ofmini-cells. However, filling the mini-cells with data still incurs apacketization delay, as does filling an ATM cell. For transmission ofvery low bit rate services, which are delay sensitive, it is onlyfeasible to include a small number of samples of the low bit rate datain the mini-cell payload, before the packetization delay becomesexcessive. For example, at very low bit rates, it may only be possibleto include 3 or 4 octet samples in a mini-cell payload before thepacketization delay in filling the mini-cell becomes too great. The CPSpacket header needs to be added to this payload, resulting in amini-cell comprising a 3 byte CPS packet header, and 3 or 4 bytes ofpayload data. Thus, for low bit rate data, a 3 byte mini-cell headerconstitutes an excessive overhead for a 3 or 4 byte data payload,resulting in poor bandwidth utilization at low data rates.

Referring to FIG. 8 herein, there is illustrated packetization ofmulti-user data from a plurality of low bit rate synchronous user datasources using a SSCS-single channel adaptation mechanism. In SSCS-SCA,each mini-cell carries data of a single user data source. For example inFIG. 8, mini-cell 800 carries data from User 1 and mini-cell 824 carriesdata from User 24. Six bytes of traffic data 825-830 are encapsulated ina service specific convergence sub-layer packet header 831 to form afirst service specific convergence sub-layer packet data unit SSCS-PDU1.The SSCS-PDU1 comprising 6 bytes of user data from User 1, plus the SSCSpacket header 831 are encapsulated in the CPS packet header 832 to formCPS packet mini-cell 801. In this case, the mini-cell 801 carries 6bytes of user information with a minimum 3 byte packet header overhead.Similarly, for further users, User 2, to User 24 whose respective dataare encapsulated in further respective mini-cells. The resulting CPSpacket mini-cells are multiplexed together to form ATM cell payloadswhich are subsequently transmitted over the physical transmission media,the node devices 300, 301, and link 303 of the network operating inaccordance with the ATM layer protocols. Using SSCS-SCA, informationfrom each single user data source is used to construct a correspondingrespective SSCS-SDU, containing that single user's data. To eachSSCS-SDU is prepended a respective SSCS packet header, to form a seriesof SSCS-PDU's. To each of these is prepended a CPS packet header to forma respective set of CPS packet mini-cells. Each CPS packet mini-cellcontains information from a single user, and so a unique ATM adaptationlayer-type 2 connection is established for the support of each user.Thus, in the CPS packet header, the user ID (UID) is thereforeassociated with one particular SSCS user. This means that every time anAAL-type 2 connection is set up for a new user, an AAL-type 2negotiation procedure (ANP) is invoked to negotiate a user ID (UID)value for assignment to that connection. Similarly, every time a user isreleased an AAL-type 2 negotiation procedure (ANP) is invoked to returnthat connection's user ID value to an unassigned state.

Whilst single channel adaptation is efficient for many service types,for some service types, in particular for low data rate services, thebandwidth utilization using SCA is inefficient.

Referring to FIG. 9 herein and hereafter generally, there is illustratedschematically another method of multiplexing a plurality of user datasources for transmission point to point across an ATM network inaccordance with the best mode for carrying out the present invention.The specific methods and processes described hereafter not to be takenas limiting to the general scope of the invention. In FIG. 9, aplurality of network users, (User 1 to User 24), each generatesynchronous user data which is used to construct a multiple channeladaptation (MCA) CPS packet mini-cell in which the payload of themini-cell represents a multiplex of the data from the multiple users. Asshown in FIG. 9, a data octet 901 from a data source of User 1 ismultiplexed with data octets from respective users; User 2, User 3 . . .User 24, to form a payload of a first SSCS protocol data unit 902.Subsequent SSCS-packet data units carrying subsequent data octets of theplurality of uses are constructed similarly. For example where each usergenerates 6 bytes of data, for each user one byte of data is included inseparate SSCS-protocol data units, SSCS-PDU's 1-6, culminating in thesixth SSCS-protocol data unit 907. In this manner, since a byte of datafrom each of the plurality of user data sources are multiplexed into asingle SSCS-protocol data unit, before a next byte of information fromthe first user is considered, the first byte of information from each ofthe plurality of data sources can be packetized without waiting for thenext byte of data to arrive from each or any user data source. Theresultant SSCS protocol data units are prepended with a CPS packetheader 908, resulting in CPS packets (mini-cells). These are multiplexedtogether to form the ATM cell payloads which are transmitted over thephysical network. Each mini-cell is incorporated into one or more ATMcells for transmission across the network.

Thus, for example in the case of 24 users, using a multiple channeladaptation mini-cell (CPS-PDU-MCA), the mini-cell 902 comprises CPSpacket header 908 and mini-cell payload (CPS packet payload) 909. Thepayload, rather than representing data from a single user, nowrepresents multiplexed data from multiple users, so the user data ismultiplexed on a frame by frame basis. Thus, if it is required totransmit user data of 24 different users from one point to anotherpoint, with each user generating an octet of data every 125 μs, whichwould be the case for example for user data generated using the knownpulse code modulation (PCM) method, then the 24 individual octet samplesfrom the 24 users can be multiplexed into the payload of a singlemini-cell. Information on the order in which the user data is packedinto the mini-cell is not carried in the mini-cell header, but istransmitted separately via the ATM adaptation layer type 2 negotiationprocedures (ANP). ANP messages are sent between two ATM adaptation layertype 2 entities (the node devices) to control assign removal, and statusof AAL-type 2 channels. Because the order in which the user data ispacked into the mini-cell payload is known at the transmission end, andtransmitted to the receiver entity in advance of transmitting themini-cells, at the receiver end, the data can be unpacked from themini-cell payload in known order and de-multiplexed into the 24 separatesets of user data.

Typical ANP messages include:

Assignment request

Assignment confirm

Assignment denied

Removal request

Removal confirm

Status poll

Status response

For example, in FIG. 3 an assignment request message is sent bytransmitting entity node device 300, and requests assignment of anAAL-type 2 channel. An assignment confirm message is sent by receivingentity, node device 301, and confirms assignment of an AAL-type 2channel. An assignment denied message is sent by an AAL-type 2 entitywhich denies assignment of an AAL-type 2 channel. A removal requestmessage is sent by an AAL-type 2 entity which requests removal of anAAL-type 2 channel. A removal confirm message is sent by an AAL-type 2entity which confirms removal of an AAL-type 2 channel. A status pollmessage is sent by an AAL-type 2 entity which polls a status of anAAL-type 2 channel. A status response message is sent by an AAL-type 2entity which responds to a status poll of an AAL-type 2 channel.Specific proposals for implementing the ANP protocol can be found inreference 2 herein. Information concerning the addition or deletion ofusers data sources in the multiplexed multi-user data package issignaled to the receiving entity using the ANP protocol as described,before the change of number of users actually occurs.

Referring to FIG. 10 herein, there is shown a way of structuring of anSSCS packet header within a CPS packet of an ATM mini-cell 1001implementing a specific method according to the present invention. Aplurality N octets of data from a plurality N user data sources aremultiplexed into data payload, service data unit packet 1002. SSCSpacket header 1003 prepended to the service data unit 1002 comprises a 2bit packet payload type field (PPT); a single bit change indicationfield (CI) 1005, a 2 bit Modulo 8 sequence indicator field (SN) 1006;and a 3 bit reserved field 1007. Using the SSCS packet header 1003 ofFIG. 10 to prepend a multi-user multiplexed data packet 1002,information from multiple low bit rate sources can be multiplexedtogether into a trunk group data frame to achieve an improved bandwidthefficiency for a given bounded cell assembly delay. Irrespective oftrunk group data frame size, the SSCS multi-channel access mechanism ofFIG. 10 can be tuned to generate service data units of lengthapproximately equal to the ATM cell payload size (maximum 48 bytes).This may yield an optimum balance between high bandwidth efficiency andelimination of potential error extensions. The size of the trunk groupis deterministic, and at all times, a receiving entity node device hasimplicit knowledge of the length of the multi-user multiplexed packetswhich it receives. However, it is possible to change the size of thetrunk group dynamically during the lifetime of a connection toaccommodate changes in the number of low bit rate users. Changes inmulti-user multiplexed data package size (ie the trunk group data frame)are signaled ahead using the ANP negotiation protocol, and the actualmoment of implementation of the change of number of users is activatedby transmission of the change indication (CI) bit signal within the SSCSpacket header in which the change occurs, such that the receiving entityknows from the ANP negotiation process what the change in the number ofusers will be, and from the received mini-cells, the timing of when thechange in number of users occurs.

Operation of the sequence indication (SI) field, continuation indication(CI) field and packet payload type (PPT) field will be described.

The 2 bit sequence indication field (SI) shown in FIG. 10 herein, isused in conjunction with the 1 bit start field sequence number (SN) inthe start field of every CPS-packet data unit, as shown in FIG. 7herein. The 2 bit sequence indication field (SI) provides a mechanismfor guarding against the loss or mis-delivery of a mini-cell. Forsuccessive mini-cells transmitted, the sequence indication field isincremented through the cycle 00, 01, 10, 11 and then back to 00. Thereceiving node device is configured to read the sequence indicationfield and check the cyclic incrementation of the sequence indicationfield. Any changes to the cycle indicate that a mini-cell has been lost.Particularly in synchronous services, it is important to ensure that anend to end phase relationship is maintained, and detection of lost ormiss-delivered mini-cells is important. Generally, loss of phase maylead to error extension, particularly for modem traffic, where the modemrequires a significant duration in order to regain synchronization, oncesynchronization is lost.

The single bit continuation indication field (CI) can be used toincorporate a large trunk group data frame, ie a long sequence ofmultiplexed users, into a plurality of mini-cells. For example, in FIG.11 herein, 160 octets from 160 user data sources respectively areassembled into a single trunk group data frame. Referring to FIG. 11herein, first to fourth successive CPS packet mini-cells 1101-1104respectively each have a respective data payload which is filled withthe data octets from the multiple users. The trunk group is transmittedin the first to fourth mini-cells 1101-1104 by filling data payload ofthe first mini-cell 1101 with octets from users 1 to 45, filling datapayload of the second mini-cell 1102 with octets from data users 46 to90, filling data payload of third mini-cell 1103 with octets from datausers 91 to 134, and filling data payload of the fourth mini-cell 1104with octets from data users 136 to 160. In the SSCS packet header 1003of each mini-cell, the single bit continuation indication field (CI) canadopt a value of 0 or 1. The value 1 is used to signify that at the endof the mini-cell containing the continuation indicator 1, the datapayload continues into a subsequent mini-cell. Thus, in FIG. 11 firstmini-cell 1101 containing continuation indicator value 1 has a datapayload part of the trunk group data frame which continues intosubsequent second mini-cell 1102. Similarly for mini-cells 1102 and1103, which also contain continuation indicator value 1. The fourthmini-cell contains a continuation indicator of value 0, indicating thatthe data payload of the fourth continuation cell does not continue intoa subsequent fifth mini-cell.

For mini-cells shown in FIG. 11 a maximum data payload of 45 octets isselected as a default condition. This value has been selected tocoincide with the suggested default length of mini-cell stated inreference 2 herein. If very large trunk group payloads are assembled ina single large mini-cell, then several successive ATM cell payloadscould be generated without any CPS protocol control information. Underthese circumstances, a single error in the data payload could lead to aprolonged error extension, thus degrading the overall error performanceof the ATM communications link. In the present method, although themaximum mini-cell payload size of 45 octets is complied with, there isno restraint placed on the maximum size of a trunk group itself whichcan be transmitted. Mini-cells having a continued payload, and having acontinuation indicator value of 1 are set at single constant length ofthe default mini-cell payload size of 45 octets. For mini-cells havingpayloads which are not continued to a successive mini-cell, the size ofthe mini-cell payload is variable to accommodate the remainder of thetrunk group data. Since the receiving entity has an implicit knowledgeof the length of the trunk group, due to prior signaling via the ANPmechanism, the lengths of both the continued mini-cell payloads, and thefinal un-continued mini-cell payload are known implicitly. However,whilst the best mode herein contemplates a mini-cell of maximum payload45 octets, the invention is not restricted to such mini-cell payloadlength. In a further specific method according to the present invention,the mini-cell payload size of a continued mini-cell may be set at anoptional maximum mini-cell payload length of 64 octets.

The two bit packet payload type field (PPT) is used both to designatethe type of packet payload, whether operation and maintenance (OAM)data, or user data, and to dynamically indicate in which mini-cell achange of number of users in the trunk group occurs. In the 2 bit packetpayload type field, the value 11 is assigned as indicating that themini-cell contains operation and maintenance signal information (OAM).In this case, the data payload of the mini-cell contains operation andmaintenance signal information, and not trunk group user data. The otherthree states of the 2 bit packet payload type (PPT) field are allocatedas follows:

00—indicates an SSCS packet which carries user information (ie trunkgroup information of the multiplexed multi-user data).

01—indicates an SSCS packet carrying user information, the same as value00 above.

Values 00 and 01 are used to alternate with each other each time thereis a change in number of users carried over a connection.

10 indicates an SSCS packet which is the first packet of a group of SSCSpackets containing data of a different number of users as theimmediately preceding SSCS packet. In otherwords 10 indicates a firstSSCS packet containing data of a new plurality of users.

Referring now to table 1 herein, there is shown a state table which maybe used to decode the packet payload type (PPT) fields when a change intrunk group structure size has occurred. In the table “same” and“inverted” imply that the PPT flag is equal to 00 or 01. Same means thatit is the same value as the last received user mini-cell, whilstinverted means it is the opposite value.

TABLE 1 P1 SN Action Same X + 1 Normal operation - no structure sizechange detected Same X + 2 Missing mini-cell detected - no structuresize change detected 10 X + 1 Structure size change detected - new sizestart this mini-cell 10 X + 2 Missing mini-cell-detected - structuresize change detected, started new mini-cell Inverted X + 2 Missingmini-cell detected - structure size change detected, started in previousmini-cell All other combinations Errored condition - take mitigatingaction as necessary, ie verify/resync at next mini-cell

Since the values 00 and 01 are both used to indicate SSCS packets whichcarry user information, but are two different states, a transition from00 to 01 or from 01 to 00 is used to indicate a permanent change in thesize of the mini-cell payload has occurred. For example. Referring toFIG. 12 herein where mini-cells A to H are transmitted, and the numberof users in a trunk group varies from 20 users, to 21 users and then to22 users, the packet payload type indicator values 00, 01 are used toindicate permanent changes of user number as follows. Firstly the changein user number from 20 users to 21 users is signaled from thetransmitter node entity to the receiver node entity in advance of thechange, using the ATM adaptation layer type-2 negotiation procedure(ANP). The number of users in the trunk group is then changed at thetransmitter end, and the new number 21, of users, are included in thetrunk group. In the first mini-cell (mini-cell D) carrying data from thenew trunk group of 21 users, the user data payload of the mini-cell ispacketized by adding the SSCS header, including in the packet payloadtype field change pulse signal, 10. At the receiver, a change pulsedecoder apparatus decodes the received SSCS packet header and determinesfrom the PPT value 10 that a change in user number has occurred in thetrunk group, starting at mini-cell D. The receiver has information inadvance of mini-cell D, of the actual number of users in the new trunkgroup, since this has been transmitted earlier using the ANP protocol.On receiving the change pulse signal PPT value 10 the receiver may thenallocate the 21 users octets to a respective 21 user end pointdestinations, taking the timing of this allocation from the change pulseindicator value 10 of the packet-payload type field.

In the next mini-cell, mini-cell E the packet payload type field (PPT)changes to the alternate value of the permanent change indicator (PCI),ie 01. The permanent change indicator PPT value 01 indicates that achange in the number of users of the trunk group has been made. Thevalue of packet payload type is maintained at the new value of permanentchange indicator PPT value, 01 for as long as the number of users in thetrunk group remains the same, in this case up to mini-cell F.

New data concerning a new number of users of the trunk group is signaledahead using the ANP protocol to the receiver, during transmission ofmini-cells D to F, to enable the receiver to set up for a new change oftrunk group user at mini-cell G. In this example, mini-cells G, Hinclude trunk data from 22 users, effective as from mini-cell G. Datafrom the new number of user data sources of the trunk group aremultiplexed into a trunk group frame which is transmitted in mini-cellsG, H and subsequent mini-cells. At the beginning of the first mini-cellcontaining the amended number of trunk group users, ie mini-cell G, thePPT field of mini-cell G contains the value 10, being the change pulseCP signal. At the receiver, the PPT field value 10 is decoded asindicating that the new number of users in the trunk group is effectiveas from mini-cell G. The decoder proceeds to de-multiplex mini-cell Gand subsequent mini-cells in accordance with the revised ANP informationindicating that there are now 22 users in the trunk group and receivedmultiplexed data octets of the 22 user data sources are sent to thecorresponding respective 22 user destinations at the receiver switch. Inthe next mini-cell following the mini-cell G containing the changepulse, the PPT field reverts to the other permanent change indicatorvalue 00.

Referring to FIG. 13 herein, a general overview of a transmission methodis shown. In step 1301, the transmitter signals ahead to the receiver,indicating a new trunk group size, using the ANP protocol. In step 1302,the number of users of the trunk group are changed and data from the newplurality of users are multiplexed into a new trunk group frame. In step1303, the trunk group frame containing the new plurality of user data ispacketized into one or more mini-cell payloads. To each mini-cellpayload is added an SSCS field header. To the first mini-cell whichcarries the first trunk group frame having a new plurality of users,there is added in the PPT field the change pulse, value 10 in step 1304.In step 1305 to subsequent mini-cells containing information from thesame trunk group frame, there is added the permanent change indicatorvalue of 00 or 01. The value 00 or 01 is selected as being a differentvalue to the previous permanent change indicator value used formini-cells carrying precious trunk group frames of the previous,different plurality of users.

Referring to FIG. 14 herein, there is shown a general overview of aprocess implemented at the receiver entity for decoding the mini-cellheader information and for allocating mini-cell data payloads to enduser destinations. In step 1401, the receiving entity receives asequence of mini-cells. In step 1402, the receiver decodes the PPT fieldin the SSCS header of the incoming mini-cells. If the decoder detects nochange in the PPT field compared to a previously received PPT field ofthe previously received mini-cell, then in step 1404, the decoderde-multiplexes the data payload of the mini-cell and switches the dataoctets of that data payload of the mini-cell to the plurality of userend destinations specified in the currently held ANP information at thereceiving entity. However, if in step 1403, the decoding receiverdetects a change in the PPT field compared to the PPT field of thepreviously received mini-cell, then in steps 1405-1408, the receivingswitch ascertains the value of the PPT field, either 00, 01, 10 or 11.

If the PPT field value is 11, then the receiving switch treats the datapayload of the mini-cell containing the PPT field value as operation andmaintenance (OAM) data in step 1409.

If the receiving switch decodes the PPT value of its currently receivedmini-cell as being 10 in step 1407, then the switch implements procedureupon change of number of users in a trunk group as illustrated in FIG.15 herein. If the receiving switch decodes the PPT field of its currentreceived mini-cell as being value 00 or value 01 in steps 1405 or 1406,then the receiving switch checks whether the previous PPT value of theeprevious mini-cell was 10, indicating a change in number of users in thetrunk group frame. If the PPT value in the previous mini-cell was 10 instep 1411, then the receiving switch treats the present mini-cell as theprevious mini-cell, and de-multiplexes its data payload in accordancewith the current information received from the ANP protocol, anddistributes the data octets of the data payload of the current mini-cellto the appropriate plurality of users indicated in the ANP protocolinformation in step 1404. However, if the current mini-cell has a PPTvalue of 00 or 01, and the PPT value of the previous mini-cell was not10, since in step 1403 it has been checked whether the PPT field haschanged from that contained in the previous mini-cell, this indicatesthat an error has occurred, and one or more mini-cells have been lostduring transmission. A lost mini-cell recovery procedure 1412illustrated in FIG. 16 is then followed.

Referring to FIG. 15 herein, where the PPT value 10 is received in thecurrent mini-cell at the receiving entity, this indicates that a changein number of users in the trunk group has occurred starting in thecurrently received mini-cell in step 1501. The receiving switch checkswhether the latest ANP information has already been implemented yet instep 1502. If the latest ANP information has not yet been implemented,then the PPT value 10 indicates the timing of the change to the newplurality of users indicated by the latest received ANP information andin step 1503 the receiving switch de-multiplexes the current mini-celldata payload and sends data octets of the new plurality of usersindicated in the ANP information to a corresponding respective newnumber of user destinations. If the latest ANP information received bythe switch has been implemented in step 1502, then an error hasoccurred.

Referring to FIG. 16 herein, where the PPT values 00 or 01 are received,and this is a change from a PPT value in the previous mini-cell, and thevalue of the PPT field in the previous mini-cell was not 10, then thisindicates that one or more mini-cells have been lost in transmission. Ingeneral, where mini-cells are lost during transmission this can causede-synchronization of certain types of apparatus, for example modems,which take a large number of cycles to re-synchronize. Loss of a singlemini-cell due to cell congestion can occur relatively commonly comparedto other cell loss mechanisms. Thus, recovery of single mini-cell losserrors may improve the performance of customer equipment attached to ATMnetworks, by avoiding loss of synchronization. Where a large number ofmini-cells are lost, for example more than one it may be thatsynchronization of modem equipment is lost in any case. In the best modedescribed herein, the PPT field can be used to recover single mini-cellloss errors as described in FIG. 16 herein. In step 1601, the 2 bitsequence indicator (SI) and the 1 bit sequence number (SN) are decodedto see how many mini-cells have been lost. In step 1602, if the switchdetermines that only one mini-cell has been lost then due to the changein value of the permanent change indicator from one mini-cell to thenext, and knowing that the previous mini-cell did not contain the value10, this indicates that the previous (lost) mini-cell must have been themini-cell in which the change of users in the trunk group occurred.Therefore, in step 1604, the receiving switch checks whether the latestANP information has yet been implemented, ie whether the switch iswaiting for a change of number of trunk group users or not. If theswitch determines that the latest ANP information has not yet beenimplemented, since the switch has knowledge that the change of user hasoccurred in the previous (lost) mini-cell in step 1603, then in step1605, the switch can continue to de-multiplex the payload of thecurrently received mini-cell and distribute data octets to the newplurality of users specified in the latest ANP information to bereceived. If in step 1602 the switch, having checked the sequenceindicator and sequence number determines that more than one mini-cellhas been lost, then an error has occurred. The error may or may not berecoverable depending upon how many mini-cells have been lost and modemequipment at the user destinations may or may not be able re-synchronizein a subsequent trunk frame in step 1606. However, due to the presenceof the permanent change indicator (PCI), the receiving user equipmentwill recover synchronization eventually. Thus, the loss of one mini-cellis fully recoverable, however the loss of synchronization after loss ofmore than one mini-cells may take a significantly longer number ofmini-cells to occur. Since the loss of a single cell is a relativelycommon occurrence, recovery from single mini-cell loss errors mayconstitute to significant advantage of the best mode described herein.The presence of the permanent change indicator ensures that even after aloss of a long burst of mini-cells, the receiving switch will alwaysknow whether the same number of users are in the trunk group or not, andrecovery of de-multiplexing to the correct end users will be possibleafter a prolonged mini-cell loss. Since changes in the number of usersof the trunk group are made dynamically in order to accommodate asmaller or larger number of low bit rate users on the connection, viathe ANP, at all stages the receiving switch has full knowledge of thecurrent and new trunk group size.

Signaling of the changes in the number of user data souces is asfollows. The ANP is used for the negotiation of a connection identifier(CID) for a newly established multiple channel adaptation connection.Thereafter, use of a CHANGE procedure is a minimal extension to the ANPprocedure required in order to negotiate a change in the number of usersbeing supported (ie the number of time slots being carried by) anSSCS-MCA connection. This procedure operates as follows:

the requesting node transmits a CHANGE REQUEST message containing acorrelation reference, the CID value for the AAL-type 2 connection, CPSservice options and UUI field conveying the details of the time slot(s)to be added or removed from the trunk group, included location(s) withinthe structure, ie the offset value(s)

if the responding node is unable to accept the requested change orchanges, it replies with a CHANGE DENIED message with a same correlationreference and CID value as the original message plus a Cause fieldcarrying the reason for denying the requested changes. The multiplechannel adaptation connection then continues with its structureunchanged.

Alternatively, if the responding node is able to accept the requestedchanges, it replies with a CHANGE CONFIRM message containing theoriginal correlation reference and UUI field.

Following ANP negotiations and agreement on a new trunk group structure,the requesting node device may implement the change. It must provide anin-band Change Indication Mechanism to accompany the change agreed viaANP so that the receiver can detect the change boundary. In this way,the end to end phase relationship between transmit and receive stationsis maintained at all times. The in-band change mechanism may be robustin the possible presence of bit error or cell loss conditions, and thusmeets at least the following minimum requirements:

1) An ability to determine a phase/start of structure size change.

2) An ability to always determine current structure size even in theevent of burst error conditions, ie it is always possible to attain fullre-synchronization even when a change is pending.

3) No error extension in the event that the change occurs in thepresence of a single cell loss/error condition.

Requirement 2) above dictates that the basic mechanism should provide apermanent indication that the change has occurred. Any structure changerelying solely on a transitory indicator could be missed completelyunder cell loss conditions, making the resultant re-synchronizationprocess complex and potentially ambiguous.

Requirement 3) above is more demanding. In the event of a packet loss apermanent change indicator will generally indicate that a change isabout to occur or has already occurred. It may not be sufficient topredict the exact phase boundary of the change. If this ismisinterpolated, in the worst case a permanent phase change will occur.To meet this requirements the in-band structure size change indicationis implemented via codes with the PPT as described herein, which provideboth a permanent and a transitory indication of the change.

The 00 and 01 values of the PPT are used to provide the permanent changeindication—the value of the PPT is flipped between these values betweensuccessive changes of the trunk group structure size. The transitoryelement is provided using the 10 value of PPT field, giving furthercorrelation of the structure size change. The PPT value is “pulsed” onceto indicate the position of the change in the trunk group frame. Forexample if the original value of the PPT is 00, then all user mini-cellswill contain this value up to the mini-cell containing the start of thestructure size change. This mini-cell alone will contain a PPT value of10, thereafter (until the next change in trunk group size is made) themini-cells will contain a PPT set to 01. The mechanism may provide asecure change indication method even in the event of cell loss.

As can be seen from the foregoing, a difference between the singlechannel adaptation mechanism and the multiple channel adaptationmechanism as described herein lies in the functionality of the SSCS. ForAAL-2 SCA, the SSCS uses information from a single source to generateeach SSCS-packet data unit whereas for AAL-type 2 multiple channeladaptation, the SSCS users information from multiple sources to generatean SSCS-packet data unit. The single channel adaptation and multiplechannel adaptation packets may be transported together on the same ATMconnection. The AAL-2 multiple channel adaptation mechanism resideswithin the SSCS sub-layer and may be implemented at no cost to otherapplications. In addition, it places minimal extra requirements on theAAL-2 negotiation procedures (ANP).

The low bit rate synchronous services supported by SCA and MCA compriseany low bit rate (64 kbit/s/s or lower) service which generates userinformation on a fixed periodic basis. For example, for 64 kbit/s/s PCM32 kbit/s/s ADPCM, and 16 kbit/s/s ADPCM, an octet of information isgenerated every 125 μS, 250 μS, and 500 μS respectively. For LD-CELP 10bits of information are generated every 625 μS. Table 2 below shows thatfor these services, by multiplexing on a trunk group basis usingSSCS-MCA a considerable increase in bandwidth utilization efficiency fora given bounded cell assembly delay can be attained compared withSSCS-SCA. The results in table 2 assume a 1 μS SSCS-PDU assembly delay.

TABLE 2 Efficiency Coding Algorithm SSCS-MCA SSCS-SCA 64 kbits/s PCM86-92% 71% 32 kbits/s ADPCM 86-92% 56% 16 kbits/s ADPCM 86-92% 39%LD-CELP 91% 25%

There are a number of potential applications than can be readilyidentified that will benefit from the use of MCA. These include PBX toPBX trunking, MSC to MSC trunking, variable PxPX sub-rate formulti-media to the desk top, and Legacy inter-working to the publicswitch telephone network (PSTN). The ability to multiplex on a trunkgroup basis as provided by SSCS-MCA together with the ability tomultiplex on the user by user basis using SSCS-SCA may significantlyenhance the applicability and flexibility of the AAL-type 2 layer.

SSCS-MCA methods described herein may enable information from multiplelow bit rate sources to be multiplexed together on a trunk group basisto achieve a high bandwidth efficiency for a given bounded cell assemblydelay. Irrespective of trunk group size, the SSCS-MCA mechanism can betuned to generate SDU's of length approximately equal to the ATM payloadsize. This may yield an optimum balance between high bandwidthefficiency and the elimination of potential error extension. At alltimes the receiving station may have implicit knowledge of the length ofthe packets which it receives. However, it is possible to change thesize of the trunk group dynamically during the lifetime of a connectionto accommodate changes in the community of low bit rate users. Thestructure size change is performed in a controlled manner through acombination of the ANP negotiation procedure and in-bandsynchronization, and so again the receiving entity has full knowledge ofthe new structure before the change is made.

The SSCS-MCA mechanism may be used to achieve optimum bandwidthutilization whilst minimizing error extension effects and enabling MCAusers to be freely multiplexed with SCA users. This can be achieve bytuning the MCA mechanism such that it produces SDU's of near optimumlength irrespective of trunk group size. The optimum packet length isequal to the free packet payload size of the CPS-PDU thus minimizingCPS-packet header overhead whilst still guaranteeing one CPS packetheader per cell to avoid error extension effects. This can be achievedin two ways:

For large trunk groups (of length greater than the CPS packet payloadsize) SSCS-MCA can segment trunk group frame the structure acrossmultiple SDU's; and for small trunk groups it is possible to concatenateseveral frames of data into one SDU.

Using SSCS-MCA recovery (without loss of synchronization) may beattainable after the loss of a single mini-cell or other errorcondition. There is no requirement for error detection or correctionover the payload information in the SSCA-MCA SDU. It is possible todetermine when there is an error in the SSCS packet header. The errorcontrol field acting over the CPS packet header is sufficient to furtherminimize any risk of possible misconnection due to error in the UUIfield.

Referring to FIG. 17 herein, there is shown an example of how a trunkgroup frame size change can be interpolated during the loss of a singlemini-cell, whilst still maintaining synchronization. In FIG. 17, thereis shown a case where a CPS packet mini-cell sequence incurs loss of aCPS packet before a change of trunk group size. The packet sequence isincremented using the sequence indicator 5, 6, 7, 0, 1, 2, 3, 4. The PPTfield values during the sequence of 8 transmitted packets is 01, 01, 01,10, 00, 00, 00, 00. The third packet is lost (packet sequenced 0), butbecause the packet, sequenced 1, has a PPT field set to 10 the receiverinfers that the lost packet, sequenced 0, did not contain a structuresize change, but the received packet sequenced 1 does contain astructure size change.

Referring to FIG. 18 herein, a sequence of eight mini-cells (CPSpackets) is transmitted. In this case, a packet containing the changepulse (CP) is lost. The packets are sequenced using the sequenceindicator and sequence number in a sequence 5, 6, 7, 0, 1, 2, 3, 4 andthe fourth CPS packet, sequenced 0, is lost during transmission. Sincethe next packet, packet 1 has an inverted permanent change indicatorcompared to the last received packet 7, received before the lost packet,the receiver can infer that a change must have occurred during the lostpacket.

Referring to FIG. 19 herein, there is shown an error recovery procedurein a case where a CPS packet mini-cell is lost after a change of numberof users in the trunk group. Mini-cells are sequenced 5, 6, 7, 0, 1, 2,3, 4 and CPS packet mini-cell 1 is lost during transmission. Sincemini-cell 1 is received, the trunk group user number size change isimplemented by the receiving switch. Mini-cell 2 is lost, but mini-cell3 has an inverted permanent change indicator (PCI value of 00 comparedto the previous permanent change indicator) PCA value 01 before thechange in trunk group size. The receiver can therefore infer that therehave been no changes in trunk group size contained in lost packet 2.

Referring to FIG. 20 herein, there is illustrated an example ofsegmentation of a large trunk group containing a large plurality of userdata from a large plurality of user data sources into several successivemini-cells. There is no restraint on the maximum size of trunk groupwhich can be multiplexed into a number of cells in the present bestmode, since the 1 bit continuation indication field (CI) can be used toassemble large trunk groups into plurality of mini-cells as hereinbeforedescribed with reference to FIG. 11.

FIG. 20 illustrates multiplexing of data from 49 user data sources intoa plurality of mini-cell packets. A user trunk group containing 49octets of user data from 49 different user data sources is multiplexedinto a plurality of mini-cells. The first mini-cell contains thecontinuation indicator (CI) value 1, indicating that the data payload ofthe mini-cell is continued into the next mini-cell 2002. The firstmini-cell 2001 includes a PPT field value 01 (this value could have alsobeen value 00, depending upon the previous inversion 00 or 01 inprevious mini-cells). The first 45 octets of data are included as thedata payload of first mini-cell 2001. The CPS packet header of the firstmini-cell 2001 includes a length indicator of 45 (LI=45). The trunkgroup data is continued into second mini-cell 2002. The continuationindicator in the second mini-cell 2002 is set at 0, indicating that thedata payload of the second mini-cell is not continued into a thirdmini-cell. The second mini-cell includes channels 46 to 49 of the userdata, ie a relatively short data payload. The length indicator in thecorresponding CPS packet header is set to 4, indicating a payload sizeof 4 octets. The next trunk group frame of 49 users is multiplexed in asimilar way in third mini-cell 2003 and subsequent mini-cells.

Referring to FIG. 21 herein, there is shown an example of multiplexingof a relatively small trunk group having a relatively small number ofuser data sources. Several successive trunk group frames can beconcatenated into a single CPS packet payload. Since packet assemblydelay is increased when successive trunk group frames are multiplexedtogether, a minimum packet assembly delay is therefore controlled byspecifying a minimum trunk group size. For example a minimum trunk groupsize of 6 users implies that it will take almost 7 successive frames togenerate an SSCS-service data unit whose length will fit into a maximumlength CPS packet payload. At a 64 kbit/s user bit rate, this implies amaximum packet assembly delay of less that 1 ms, which is satisfactoryfor many applications. The maximum packet assembly delay can beincreased or decreased by defining a minimum trunk group sizeaccordingly. There is no net penalty in delay with this approach.Concatenation of structures into an SSCS protocol data unit first isequivalent to multiplexing packets by the CPS into an ATM cell.

The number of frames that can be bound into a single SSCS data unit isdependent upon the current trunk group structure size. It may becalculated as the integer division of the maximum CPS packet payloadsize (45 octets) by the trunk group size. The CPS packet header overheadis thus minimized whilst the resulting protocol data unit does notexceed the maximum packet length limit. For example with a 6 channeltrunk group this implies that 7 successive frames can be concatenatedtogether. The efficiency of the SSCS-MCA connection therefore varieswith a structure size, but in all cases the utilization is significantlyhigher than that attained for a similar SSCS single channel adaptationconnection. Further, although the size of the packet varies with thegroup structure size, its size is completely deterministic and thereceiver always has implicit knowledge of its length.

Abbreviations AAL ATM adaptation layer AAL-type ATM adaptation layertype 1 1 AAL-type ATM adaptation layer type 2 2 ANP ATM negotiationprocedure ATM asynchronous transfer mode CI continuation indicationfield (1 bit) CP change pulse CPS common part sub-layer CRC cyclicredundancy data LI length indication MCA multiple channel adaptation OAMoperation and maintenance PCI permanent change indicator PDU protocoldata unit PPT packet payload type SCA single channel adaptation SDUservice data unit SI sequence indication field (2 bits) SN sequencenumber (1 bit) in start field SSCS service specific convergencesub-layer UID user identification UUI CPS user to user indication

REFERENCES

[1] Copies of the ATM standards protocols are available fromInternational Telecommunications Union (ITU), Sales and MarketingService, Place des Nations, CH-1211, Geneva 20, Switzerland, telephone+41 22 730-66666 or from the ATM Forum, 2570 West El Camino Real, Suite304, Mountain View, Calif. CA90404, USA.

[2] Draft ITU-T Recommendation I.363.2 “B-ISDN ATM Adaptation LayerType-2 Specification” (Madrid 1996), Recommendation I.363.2 (November1996), available from International Telecommunications Union.

What is claimed is:
 1. A method of communicating user data of aplurality of user data sources over a communications network, saidmethod comprising the steps of: multiplexing a data sample from each ofa plurality of user data sources to produce a frame of user data, saidframe containing data of each of said plurality of user data sources;and assembling said frame into a data payload of at least oneasynchronous transfer mode mini-cell; including a protocol header signalin a said mini-cell, to indicate a change in number of user data sourceswhose data is assembled into a said at least one mini-cell; and using apacket payload type field of a service specific convergence sub-layerheader of an asynchronous transfer mode mini-cell to indicate change ofnumber of said user data sources whose data is carried in a series ofsaid mini-cells.
 2. A method as claimed in claim 1, wherein said frameis of a length greater than the payload length of a said mini-cell.
 3. Amethod as claimed in claim 2, wherein a said frame is partitioned to runconsecutively across a plurality of said mini-cell payloads.
 4. A methodas claimed in claim 3, comprising the step of: assembling a respectiveprotocol header to each of said mini-cells, said protocol headercomprising a continuation indicator signal indicating whether or notsaid frame continues beyond a length of said mini-cell.
 5. A method asclaimed in claim 4, wherein said continuation indicator signal comprisesa single bit field.
 6. A method as claimed in claim 5, wherein a saidmini-cell comprises an asynchronous transfer mode adaptation layer-type2 common part sub-layer packet.
 7. A method as claimed in claim 6,wherein said continuation indicator signal comprises an asynchronoustransfer mode adaptation layer-type 2 header.
 8. A method as claimed inclaim 7, wherein said continuation indicator signal comprises anasynchronous transfer mode adaptation layer-type 2 service specificconvergence sub-layer field.